1. Field of Invention
This invention pertains to microscopes and more particularly a method and apparatus for focusing a microscope automatically by projecting a preselected image on the sample surface.
2. Description of the Prior Art
Numerous schemes and devices have been proposed in the past for focusing microscopes automatically. Generally these schemes may be grouped into two major categories. One category makes use of the so-called photoelectric method while the second uses an image detection method. The photoelectric method comprises focusing the image of a light source (such as for example a He-Ne laser) on the sample surface and collecting the reflected light from the surface with a photodetector. The photodetector is located in the image plane of the sample and autofocusing is achieved by maximizing the light reaching the photodetector. A major deficiency of this method is that it requires a highly reflective and relatively flat sample surface to provide sufficient signal for the detector. In the second method, the actual image of the sample surface is scanned by a TV camera or photodetecting diode array to determine the image contrast. Autofocusing is achieved at the point of maximum contrast. A major deficiency of the second method is that the sample surface must have some inherent contrast. For example, it is difficult to use this method on sample surfaces which lack significant surface texture such as flat glass or a metal plate.